JPEG XR
| Filename extension |
.wdp |
|---|---|
| Internet media type |
image/vnd.ms-photo |
| Developed by | Microsoft |
| Type of format | graphics file format |
Windows Media Photo (WMPhoto) is a high compression still image format for continuous tone photographic images developed by Microsoft as a part of the Windows Media family. It can support both lossy as well as lossless compression, and is the preferred image format for Microsoft's XPS documents.
Description
Windows Media Photo is an image codec which gives a high dynamic range image encoding while requiring only integer operations (with no divides) for both compression and decompression. It supports monochrome, RGB, CMYK and even n-channel color representation, using up to 16-bit unsigned integer representation, or up to 32-bit fixed point or floating point representation, and also supports RGBE Radiance. It may optionally include an embedded ICC color profile, to achieve consistent color representation across multiple devices. An alpha channel may be present for transparency, and EXIF and XMP metadata formats are supported. The format also supports multiple images per file. The format allows decoding part of an image, without decoding the entire image. Full decoding is also unnecessary for certain operations such as cropping, downsampling, horizontal or veritical flips, or cardinal rotations.
Compression algorithm
Windows Media Photo uses a reversible color space conversion, a reversible lapped biorthogonal transform and a non-arithmetic entropy encoding scheme. The transform operation needs 3 multiply and add operations and 7 add or shift operations at the highest quality level and in the highest performance mode, 1 multiply and add operation and 4 add or shifts are required per pixel, giving the codec a high performance. Windows Media Photo processes images in 16x16 macroblocks.
The colorspace transform uses non-division integer operations, thereby making it entirely reversible. This allows support for both lossy as well as lossless encoding, by using appropriate quantizers.
WMPhoto supports various color formats and multiple fixed and floating point numerical representation, thus giving a wide range of compression options. To remain compatible across various devices, it differentiates Basic and Advanced formats. While digital photography needs are satisfied by the Basic format, 3D rendering or advanced image processing scenarios require the Advanced format.
Microsoft claims that Windows Media Photo offers a "perceptible image quality comparable to JPEG 2000 with computational and memory performance more closely comparable to JPEG and delivers a lossy compressed image of better perceptive quality than JPEG at less than half the file size, and that the lossless compression compresses images 2.5 times".
Container format
WMPhoto uses a TIFF-like file container to store image data in a table of Image File Directory (IFD) tags. A WMPhoto file contains image data, an optional alpha channel data, WMPhoto metadata, optional XMP metadata stored as XML, and optional EXIF metadata, in IFD tags. The image data is a contiguous self-contained chunk of data. The optional alpha channel, if present, is compressed as a separate image record, enabling decoding of the image data independently of transparency data in applications which do not support transparency.
See also
External links
- Windows Media Photo Specification (describes container format and API only; not an image format specification)
- CNET News Article